The short answer is that you can add new things to the XBox version of the game, but usually it is just easier to say "no" and so often we do. You're on the right track in that you have your box set up to be modded, but you'll need to recreate the batch files that compile source files into the .lvl files used by the game.

Your memory limitations are more stringent than the PC version allows. I can't give you a step-by-step myself since I've never changed the batch files (nor do I own an Xbox and as such this is something I couldn't care less about), but from what I understand you should have at least a cursory knowledge of how SWBF2 mods on PC work and what the munge (compile) process entails.

Feel free to take a look through this topic here and garner what you can from it - the point remains that you will need to have a basic knowledge of working with SWBF2 mods, because the majority of what you're talking about will be significantly easier to do with creating new .lvl files in whole or in part to either add in their entirety or to add in chunks into preexisting .lvl files (with hex-editing, as you mention) . If you are open to suggestion I would offer these:
-Do a little bit of SWBF2 PC modding (at least play around with the tools and learn how to make sides)
-Send a message to the user who created the abovementioned topic and see if he's willing to share any information with you.